Various multiple-access technologies may be used for cellular communications. A first group of these technologies consists of narrowband channelized technologies such as the Frequency-Division Multiple Access (FDMA) technology and the Time-Division Multiple Access (TDMA) technology. In a FDMA communication system each user is assigned for the duration of a call to a first specific frequency sub-band, or channel, of the bandwidth reserved for up-link communications (from a mobile station to a base station) and to a second frequency sub-band, or channel, of the bandwidth reserved for down-link communications (from a base station to a mobile station). In a TDMA system each user is assigned to a different time slot and is entitled to access the entire reserved sub-bands.
A second group of multiple-access communication technologies consists of wideband channelized technologies. Among these, the Code-Division Multiple Access (CDMA) technology has been widely adopted as a standard. CDMA allows each user to use the entire bandwidth for the complete duration of a call.
CDMA is a spread spectrum technology which means that the information contained in the information signal is spread over a much greater bandwidth than that of the original signal. In the Direct Sequence Spread Spectrum (DS-SS) technology, the information signal of data rate Tb is multiplied in the transmitter by a pseudo-random binary sequence, the code sequence, of clock period T, the so-called chip period, where Tb>>T. This has the effect of increasing the bandwidth of the signal by the ratio Tb/T. The spread signal is then transmitted over the wider band with a reduced power spectral density relative to a corresponding de-spread signal. The code sequence is independent of the information signal and is known to the transmitter and the receiver.
At the receiver, the received wideband spread spectrum signal must be de-spread in order for the information signal to be recovered. De-spreading is achieved by correlating the spread signal with an exact replica of the code sequence used in the transmitter. The replica must be synchronized with the received spread signal. The replica of the code sequence is locally generated at the receiver and must be aligned and synchronized within one chip of the received spread signal.
Code synchronization may be performed in two stages: a code acquisition followed by a fine code tracking. Acquisition reduces the alignment timing offset between the received spread signal and the locally generated code sequence to less than a chip period. Tracking aligns and maintains the two signals synchronized to a finer precision.
In a real communication environment such as urban and suburban areas, radio signals are reflected and scattered off various objects along the transmission path between the transmitter and the receiver. Therefore the spread signal, mentioned above, encounters multipath when transmitted from the base station to the mobile station. In addition, phase cancellation of signals following different paths may cause severe fading and may lower the received signal power. However CDMA provides robust operation in such fading environments. Indeed CDMA takes advantage of multipath fading to enhance communication and voice quality. For this purpose, a rake receiver is present in each mobile station and allows selecting the strongest multipath signals incoming from the base station. Transmission delays are estimated for the strongest multipath signals and the estimated delays are assigned to specific “fingers” of the rake receiver. A finger is a processing element that correlates the received spread signal with the replica of the locally generated code sequence on the basis of the estimated time delay assigned to the finger. The fingers' outputs are then weighted and then coherently combined to produce an enhanced signal. Thus, the multi-path nature of the channel is used to create a diversity advantage in CDMA.